In systems of this type it is desirable, particularly in the case of loads such as television receivers operating under high voltage, to provide full electrical separation between that operating circuit and the control member, especially when the latter is manually actuated. At the same time, the user handling the control member must have reliable ways of ascertaining whether the desired effect, i.e. opening or closure of the load-operating circuit, is being achieved. Thus, it is known to use a light shutter in combination with a semi-transparent reflector for diverting a portion of the detector-activating light beam toward a display area as a visually observable signal indicating that the shutter is open and the detector is being irradiated. Since, however, the beam usually originates at a weak source of radiation such as a light-emitting diode, its passage through a semireflector significantly attenuates the available light to be used for activation and monitoring purposes. A significant loss of light also occurs in systems of the type described in British Pat. No. 1,418,318 where the operator's finger is used to reflect luminous radiation from a source to a photodetector. The use for this purpose of a mirror carried on a displaceable switch member, without any luminous signal indicating that member's position, is suggested by German published specification No. 2,516,171.